The invention relates to a method for manufacturing a welded shaped body of platinum material dispersion-hardened by finely divided small particles of base metal oxide, especially such a body provided with at least one inside wall, e.g. a tube. The invention also relates to a shaped body, especially a tube, made by the method and to the use of such a tube.
It is known from the prior art (see German Patent Application DE-OS 15 33 273) that platinum, palladium and rhodium as well as alloys thereof containing a small proportion of one or more dissolved base metals can be subjected to internal oxidation to create a dispersion-hardened material.
In this process a small quantity of at least one base metal capable of forming a stable heat-resistant compound is alloyed into platinum, palladium or rhodium or alloys of these metals containing one or more other metals of the platinum group, after which the alloying additive is transformed to the heat resistant compound which is dispersed through the alloy. Examples of suitable base metals are chromium, beryllium, magnesium, aluminum, silicon, the rare earths, thorium, uranium and metals of the first, second and third subgroups of the periodic table, calcium to nickel, strontium to molybdenum and barium to tantalum. The heat-resisting compound can be an oxide, a carbide, a nitride, a silicide, a boride, a sulfide or any other heat resisting compound which can be formed by interaction between a gaseous phase and the base metal.
It is also known from the prior art (see German Patent Application DE-OS 15 33 273) that sheets of a metal of the platinum group or alloys thereof can exist in dispersion-hardened form with an addition of the above nonmetallic compounds.
However, it is also known ("Mechanical Properties of Metallic Composites," edited by Shojiro Ochiai, 1993, pages 352-353) that, during welding of oxide-dispersion-hardened platinum materials, the oxide dispersion hardening is largely lost. This is due to the melting process during welding which leads to agglomeration and washing-out of the oxide dispersoids and thus to loss of the favorable characteristics. As also discussed, this has led to attempts to fabricate items from the platinum materials without using hot welding techniques.
Usually platinum materials with fine-grained equiaxial microstructure are used in the manufacture of components. This microstructure is created by forming (forging or rolling, for example) a smelted and cast bar and then subjecting it to recrystallization annealing. If the material is subsequently welded, the microstructure developed in the weld after solidification of the metal is more like the undesired microstructure in cast bars than with the fine-grained microstructure of the rest of the material, which was obtained by the recrystallization annealing. The microstructure can be homogenized (that is, a more uniform structure can be formed) by forming the weld together with the rest of the material, and this becomes apparent after recrystallization annealing treatment, in that the formed and recrystallized material of the weld corresponds substantially to the rest of the material.
It was not possible heretofore to apply the welding method to oxide-dispersion-hardened materials without losing the specific characteristics of dispersion hardening.
Since the welding process leads to washing-out of the dispersoids, as already explained hereinabove, the weld differs fundamentally from the rest of the material.
On the one hand, the hardening effect of the dispersoids will no longer be present. On the other hand, the microstructure (grain size) in the largely oxide-free weld will become substantially coarser than in the rest of the material during an annealing treatment or during service at high temperature. (The presence of dispersoids leads to considerable stabilization of the grain structure). Even after forming and annealing treatment, the coarsened grain size in the weld leads among other problems to increased corrosion susceptibility, since corrosion attack takes place mainly along the grain boundaries.
Furthermore, it was not possible heretofore to achieve a high proportion of internal oxidation in relatively thick semifinished products of platinum materials, especially with thicknesses of several millimeters. For this reason such semifinished products had to be made from a material which already contains the oxide dispersoids and thus suffers from the above-mentioned problems during welding.
It is therefore an object of the invention to eliminate at least partly the aforesaid disadvantages by means of a novel method, a novel shaped body and a use thereof.